The present invention relates generally to the processing of optical signals in nonlinear optical frequency mixers, and in particular to the alleviation of the group velocity mismatch (GVM) occurring between interaction waves in such mixers.
Development of high capacity optical networks has accelerated because of emerging demand for world-wide communications. Information, interactive multimedia service, electronic commerce, and many other services are efficiently delivered online through the Internet. Optical fiber communication serves as the enabling technology to realize those Internet activities. Today, several tens of gigabits-per-second of data traffic are carried over many thousands of kilometers through optical fiber communication systems.
Transmission of high capacity data and, more importantly, the management of that high capacity data are the keys to the realization of such global optical-fiber-based networks. This rapid evolution in communication systems is creating enormous demands for optoelectronic components with capabilities beyond those currently available. In particular, the requirements push some theoretical limitations of transmission of optical signals.
Today""s optical communication systems rely on wavelength division multiplexing (WDM) as well as time division multiplexing (TDM) techniques to send optical signals in the form of pulses through optical fiber. The pulses are designed with pulse widths as narrow as 3xc3x9710xe2x88x9212 s and the trend to narrower pulses and higher rates continues. One of the main physical limits to our ability to reduce the pulse width even further is the basic phenomenon of pulse lengthening due to the dependence of its group velocity on frequency. This phenomenon, called group velocity dispersion (GVD), affects every mode of light, with the exception of solitons and is often defined by the relation:
Dxe2x89xa1Lxe2x88x921(dT/dxcex), 
where T is the pulse transmission time through length L of the fiber and xcex is the wavelength of the light. This definition is related to the second order derivative of the propagation constant xcex2(xcfx89) of the mode with respect to its angular frequency xcfx89by:       D    =                  -                              2            ⁢            π            ⁢                          xe2x80x83                        ⁢            c                                λ            2                              ⁢              (                                            ⅆ              2                        ⁢            β                                ⅆ                          ω              2                                      )              ,
where c is the speed of light in vacuum. Meanwhile, group velocity vg is defined as:       1          v      g        =                    ⅆ        β                    ⅆ        ω              .  
When a light pulse contains several wavelength components, GVD causes these to migrate within the pulse envelope producing a xe2x80x9cchirpxe2x80x9d and it also causes the pulse to broaden. In particular, the chirp causes the longer wavelengths to migrate to the front of the pulse envelope while the shorter wavelengths recede to the back. The effects of GVD are frequently expressed in terms of a group velocity mismatch (GVM) describing the rate at which pulses at different wavelengths slip off each other.
The prior art contains many teachings related to compensation of pulse broadening occurring when pulses travel through fiber by phase conjugation. In these schemes, a pulse travels a certain length of fiber and broadens while accumulating a chirp. A phase conjugator reverses the chirp of the pulse, typically by a nonlinear mixing operation relying on a nonlinear optical material exhibiting a third order susceptibility "khgr"(3). The chirp reversed pulse travels through another length of fiber and experiences recompression. The recompression occurs because the longer wavelengths flipped to the back of the pulse will move forward and the shorter wavelengths flipped to the front of the pulse will move to the back.
In addition to the use of nonlinear materials for phase conjugation based on "khgr"(3), nonlinear optical materials having a second order susceptibility "khgr"(2) are also used in optical frequency mixers to perform various mixing functions including second harmonic generation, difference frequency generation, sum frequency generation, parametric generation or parametric amplification. These functionalities can be used in an all-optical network at nodes for switching optical signals in different wavelength channels in different directions without ever converting the optical signals into electronic form. In addition, nonlinear optical mixers can be used to switch optical signals between different optical carrier wavelengths, either within the immediate network or when transferring to a neighboring network. Such wavelength switches can be used to build wavelength interchangers or wavelength interchanging cross-connects. More information about such switches can be found in S. J. B. Yoo, xe2x80x9cWavelength Conversion Technologies for WDM Network Applicationsxe2x80x9d, Journal of Lightwave Technology, Vol. 14, No. 6, June 1995, pp. 955-66 as well as U.S. Pat. No. 5,825,517 to Antoniades et al. and the references cited therein.
The effects of GVD on short pulses, and especially on ultra-short pulses on the order of picoseconds, interferes not only with the propagation of such pulses through fiber but also with efficient nonlinear wavelength mixing of such ultra-short pulses. U.S. Pat. No. 5,815,307 to Arbore et al. and U.S. Pat. No. 5,867,304 to Galvanauskas et al. teach the use of chirped gratings to take advantage of second order susceptibility "khgr"(2) of the nonlinear material to adjust the shape of pulses. For example, Arbore et al. teach how to compress pulses during second harmonic generation (SHG) by taking advantage of the principles of GVD and nonlinear optical frequency mixing. To achieve efficient frequency conversion these devices employ quasi-phase-matching (QPM) to counteract the phase slip between the generating or pumping light and the generated or converted light as these two interaction waves propagate through the nonlinear optical material. In contrast to GVD, the phase slip is due to the fact that optical signals of different wavelengths, e.g. the pumping wave and the frequency doubled wave experience a different index of refraction in the nonlinear optical material. Thus, there is a phase velocity mismatch between the interaction waves. The QPM grating is employed in the nonlinear material to prevent the phase slip occurring between the generating and generated light signals or interaction waves due to phase velocity mismatch. Thus, by keeping the interacting waves in phase, QPM ensures efficient frequency mixing between the interaction waves.
Unfortunately, the effects of GVD are felt in nonlinear mixing processes irrespective of the type of nonlinear mixing process and phase matching technique used. GVD effects are especially pronounced when the interaction waves are short pulses and have very different wavelengths. In those situations a substantial walk-off is produced between the interaction waves over very short distances and the nonlinear mixing process stops.
The prior art describes several systems and devices which contend with dispersion problems. For example, U.S. Pat. Nos. 5,369,519 and 5,224,194 to Islam teach the use of a nonlinear material with negligible walk-off to achieve all-optical timing restoration function in optical switching and transmission systems. The negligible walk-off is realized by a hybrid solution that consists of a nonlinear chirper followed by a dispersive line. The scheme can be characterized as a hybrid solution, which needs a delay line with a dispersion sign different from the nonlinear chirper. In U.S. Pat. No. 5,696,614 Ishikawa et al. provide an optical wavelength multiplex transmission method to realize an optical communication system of an increased capacity which is not influenced by crosstalk by four-wave mixing (FWM). This patent also describes a dispersion compensation method for the WDM transmission link. Unfortunately, none of these references teach compensation for group velocity mismatch (GVM) in nonlinear frequency conversion based on material second order susceptibility "khgr"(2).
In view of the above, it would be a significant advantage over the prior art, to provide nonlinear optical mixers which are compensated for GVM. Specifically, it would be an advance to compensate for GVM effects between short-pulsed signals over sufficiently long distances to increase the efficiency of nonlinear mixing using material "khgr"(2) susceptibility between the interaction waves and to enable high bit-rate time-sequential data streams as required, e.g., for TDM networks.
In view of the above, it is a primary object of the present invention to provide a compensated nonlinear optical frequency mixer and a method to compensate nonlinear optical frequency mixers for the effects of GVM.
It is another object of the invention to provide for high conversion efficiencies of short pulsed interaction waves in nonlinear mixing processes relying on material "khgr"(2) susceptibility by compensating for walk-off between the interaction waves.
It is yet another object of the invention to provide for GVM induced walk-off compensating structures in nonlinear optical mixers which are easy to make and integrate into such mixers.
Still another object of the invention is to ensure that the GVM compensating structures and methods are compatible with most nonlinear mixing processes including second harmonic generation, difference frequency generation, sum frequency generation, parametric amplification, and parametric generation.
These and numerous other advantages of the present invention will become apparent upon reading the detailed description.
The present invention provides a compensated nonlinear optical frequency mixer for compensating the walk-off produced by group velocity mismatch between a first interaction wave and a second interaction wave. The compensated mixer has a first mixing region in which the interaction waves participate in a non-linear optical mixing process. The walk-off occurs between the first and second interaction waves in this first mixing region because the interaction waves have different wavelengths and hence different group velocities. The compensated mixer is equipped with a frequency selective coupling and time delay structure located after the first mixing region for eliminating the walk-off produced between the interaction waves in the first mixing region. A second mixing region is located after the frequency-selective coupling and time delay structure, such that when the waves emerge in phase (no walk-off) from the frequency selective coupling and time delay structure they continue to interact efficiently in the second mixing region.
The frequency-selective coupling and time delay structure has a first directional coupler and a second directional coupler. In one embodiment a first arm for receiving the first interaction wave and a second arm for receiving the second interaction wave are positioned between the first and second directional couplers. The first arm is longer than the second arm by a re-synchronization length. The re-synchronization length is selected such that the first interaction wave and the second interaction wave are recombined in phase or very nearly in phase at the second directional coupler. In another embodiment, a phase-insensitive compensating arm for receiving the first interaction wave is positioned between the first and second directional couplers. In yet another embodiment, the frequency selective coupling and time delay structure is equipped with a tunable phase shifter.
The compensated nonlinear optical frequency mixer of the invention can perform any of the known frequency mixing operations. Specifically, the frequency mixer has a "khgr"(2) susceptibility in the first and second mixing regions. Therefore, the frequency mixer can support any nonlinear frequency conversion operation based on the "khgr"(2) susceptibility. For example, the mixer can support difference frequency generation, sum frequency generation, second harmonic generation, optical parametric generation and amplification.
The frequency mixer can take advantage of a quasi-phase-matching grating for phase matching the nonlinear frequency conversion. In fact, either the first or second mixing regions or both can be equipped with a quasi-phase-matching grating. The length of the first mixing region is such that the walk-off is less than 180 degrees out of phase. Depending on the total conversion efficiency desired, the number of mixing regions and interposed frequency-selective coupling and time delay structures can vary. Conveniently, the frequency mixer is designed such that the mixing regions and the frequency-selective coupling and time delay structure or structures are integrated in a single substrate.
The method of the invention is used for compensating the nonlinear optical frequency mixer for walk-off due to group velocity mismatch between first and second interacting waves. The method calls for providing the first mixing region where the walk-off between the interaction waves occurs and for providing a frequency-selective coupling and time delay structure after the first mixing region. The frequency-selective coupling and time delay structure is adjusted to eliminate the walk-off which occurred between the first and second interacting waves. A second mixing region is provided after the frequency-selective coupling and time delay structure. The adjusting of the frequency-selective coupling and time delay structure can include adjusting a propagation delay between the first and second interaction waves. Alternatively, the adjusting step can including adjusting a phase relationship between the first and second interaction waves. In any event, the adjustment is performed such that the first and second interaction waves emerge from the frequency-selective coupling and time delay structure in phase or very nearly in phase.
A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.